In my U.S. Pat. No. 4,198,955 the disclosure of which is incorporated herein by reference, there is described a solar energy collection system using compound parabolic concentrators (CPC's). A CPC collector is characterized by a compound parabolic shape which concentrates solar energy received through the aperture of the parabola onto a tube through which fluid to be heated is passed. The parabolic shape is determined by the relationship of the concentration ratio (C), i.e., the ratio of the transverse width of the aperture to the outer circumference of the tube, to the acceptance angle (.theta.), which is the angle to the axis of the parabola within which light rays are concentrated on the tube. The relationship is EQU C=1/sin .theta.
In the aforementioned U.S. Patent, there is described a solar energy collection system which is comprised of modular solar collectors. Each module comprises a plurality of elongate envelopes physically joined together in fixed immovable relationship to each other in an integral body structure and a transparent cover sealingly joined to the body structure. Each envelope is evacuated and has a CPC-shaped reflective inner surface. A tube having a selectively absorbing surface for selectively absorbing energy having predetermined wavelengths passes sequentially through the envelopes. The tube conveys fluid to be heated to the module and removes heated fluid from the module.
For ease of fabrication and assembly and for strength reasons, we have extensively used metals, notably steel, in constructing the modular system. A typical construction is illustrated in my copending U.S. application Ser. No. 101,808 filed Dec. 10, 1979, now U.S. Pat. No. 4,309,984, the disclosure of which is incorporated herein by reference.
The use of steel as the material of construction of the absorber tube has led to the formation of corrosion-generated hydrogen in the heat conveying fluid. This hydrogen permeates the steel tube into the evacuated envelope, thereby increasing the pressure therein. The loss of vacuum in this way is highly undesirable, since the efficiency of the collector is significantly diminished at lesser vacuum levels. Tests have determined that gettering of the evacuated space to remove the hydrogen is not a satisfactory solution to the problem. In addition, the hydrogen permeation causes embrittlement of the tube and eventual fatigue cracking.
This problem is not limited to the solar energy field. For example, the nuclear industry continues to experience serious problems in heat exchanger cores due to the cracking of condenser tubes particularly where the metal tube has experienced severe cold working during fabrication and assembly operations. The hydrogen apparently alters the composition of the alloy steel at specific stress areas resulting in a modification of the physical characteristics of the capability of the steel to sustain the dynamics of operational loads. The result is hydrogen-initiated cracking.
Numerous methods have been attempted to deal with this hydrogen problem in various industries. They range from the use of high alloy steels to the use of additives to the water transport fluid. All attempts to date have failed to deal effectively with the problem.